Variable-displacement screw-type compressor

ABSTRACT

The invention relates to a variable-displacement screw-type compressor with at least one main rotor and at least one subsidiary rotor which are fitted in the same housing, mesh together and convey a medium to be compressed from an inlet to an outlet, in which the inlet is bounded by at least one housing segment fitted to slide in the housing which follows the shape of the rotor casings on its sealing side facing the rotors. At least one housing segment is guided and movable transversely to the rotor axes.

TITLE OF THE INVENTION BACKGROUND OF THE INVENTION

The invention refers to a variable displacement screw-type compressor.

In installations using a compressed medium as the pressure gas, it isoften necessary to regulate the pressure prevailing in the installation.This is either done by bleeding a certain amount of pressure from theinstallation or returning it via the bypass, or by changing the feedrate of the pressure gas.

If the compressor used in such an installation is a screw-typecompressor, the feed rate may be adjusted by changing the number ofrotations of the screw-type compressor. However, this adjustment has itslimit where too small numbers of rotation of the screw-type compressorcause the efficiency of the screw-type compressor to fall below anacceptable value.

Since the bleeding and the bypassing of the pressure gas causeunacceptable energy consumption, anyway, and, on the other hand, a broadadjustment range is imperative, screw-type compressors have beendeveloped, in which the feed rate provided by the screw-type compressorcan be adjusted by the compression ratio of the screw-type compressor.

DE-05 25 26 175 shows a screw-type compressor with a variable volumetriccapacity, wherein two meshing rotors are arranged in a housing. In thisscrew-type compressor, a return flow channel extends in parallel to therotor axes of the screw-type compressor, which channel may becommunicated with the interior of the housing through closable openings.In the open state, the openings spaced in the axial direction of therotors allow for a return flow of the medium to be compressed throughthe return flow channel to the intake side of the screw-type compressor.Thereby, the compression of the medium flown in between the screws andthe inner wall of the housing starts sooner or later, depending on theopening state of the openings so that the feed rate provided by thisscrew-type compressor is variable. However, the openings, the ends ofwhich at the overflow channel side may be closed by means of a controlpiston, are also open towards the interior of the housing when in theirclosed state, whereby they form overflow pockets that cause a returnflow of the medium to be compressed even at a delivery volume of 100%.Therefore, the screw-type compressor has a poor efficiency.

DE-PS 35 16 636 on which the precharacterizing part of claim 1 is based,discloses another variable displacement screw-type compressor. In thesame, a primary and a secondary rotor are provided in a housing. In thisscrew-type compressor, the medium to be compressed is conveyed from aninlet channel to an outlet channel, the inlet channel having two housingsegments displaceable along the rotor axes. The housing segments eachextend in the housing over the entire rotor length and are floatinglysupported at one end. The pressures occurring transversely to thelongitudinal extension of the housing segments and being receivedthrough guiding paths, however, cause bending loads on the housingsegments. In operation, displacing the housing segments towards theoutlet channel causes a channel wall defining the inlet channel to bedisplaced such that the inlet channel is prolonged and the compressionstarts later. Due to the later start of the compression, the compressionratio, and thereby the feed rate of the screw-type compressor, ismodified.

To make even small changes in the displaced volume, it is necessary todisplace the respective entire housing segment extending over the fullrotor length. The unilaterally floating support of the housing segmentsnecessary therefor, however, has the disadvantage of a high productioneffort. Further, the control segments that extend through the pressureside of the screw-type compressor are induced to vibrate by the highpulsating pressures prevailing at the pressure side of the screw-typecompressor, the vibrations causing the guiding paths to become unsteady.Finally, it is another disadvantage that the housing segments cause agreat structural length of the screw-type compressor and impose a limitto the freedom of structural design on the pressure side of thescrew-type compressor.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a variable displacementscrew-type compressor that can be operated with little wear, has a highefficiency and is convenient to produce.

The object is solved, according to the invention, with the features ofclaim 1.

According to the invention, the at least one housing segment is guidedin a well of the housing to slide transversely to the rotor axes. Sincethe pressure acting on each housing segment causes a resultant pressureforce effective almost in parallel to the direction of displacement ofthe housing segment, the bearings of the housing segments are loadedonly to a small extent. This not only leads to a reduced wear of thescrew-type compressor, but also to a facilitated production thereof,since one may omit a complex support of the housing segment. Since eachhousing segment has its sealing side facing the rotors formed with ageometry following the course of the envelope of the rotors and sincethese sealing sides can always be positioned precisely, the screw-typecompressor has a high efficiency.

The slidableness of the housing segment transversal to the rotor axesalso provides for a short structural length and the freedom inconstructing the front ends of screw-type compressors is increased.Thus, transversely slidable housing segments can therefore be wellimplemented in twin screw compressors.

In order to adjust a multi-stage adjustment of the feed rate, aplurality of housing segments are arranged in succession in the axialdirection of the rotors. Preferably, each housing segment has a lengthin the axial direction of the rotors that corresponds to the axialdistance of the contact points of two meshing pairs of teeth. Since thecompression only starts when a compression chamber formed by the toothflanks and the inner housing wall is closed, a finer and, due to thehigher production effort, a smaller gradation by means of housingsegments is not necessary. Nevertheless, a fine adjustment of thedisplacement rate may be obtained in housing segments of this design bydisplacing one housing segment more or less so that the inlet channelshows a throttle effect regardless of its effective length.

If the width of each housing segment transverse to the axial directionof the rotors is dimensioned such that the change in the inlet channelvolume caused by the displacement of each housing segment corresponds tothe volume of a compression chamber defined by two tooth flanks of therotors, throttle effects causing an unfavorable efficiency, can beavoided by adjusting the feed rate in correspondence to the discretegradations of the rotor. It is true, as described above that throttleeffects may be desirable for obtaining a continuous band width of feedrates, however, these shall only occur when they are set intentionally.

By providing each housing segment with two stop surfaces that limit themovability of the housing segments towards the rotors and may preferablyabut against two abutment surfaces extending in parallel to the well, itis achieved that the distance of the housing segments to the rotors canbe observed precisely. Precisely maintaining the distance between thehousing segments and the rotors guarantees a high efficiency.

Preferably, the housing segments are biased by means of springs so thatsingle action actuators (acting in one direction) suffice to displacethe housing segments. Preferably, such actuators are control camsarranged on a common cam shaft and displacing the housing segments. Adisplacement by control cams is not only cost-effective, but moreoverguarantees that the position of all housing segments that determine thegeometry of the inlet channel can be adjusted with precise coordination.Further, the control cams make it possible to move at least one of thehousing segments into a position in which it acts as a throttle arrangedin the inlet channel. When the springs are biased such that they forcethe housing segments into their raised position, in which they bound thefeed channel, the precise observation of the closing position of thehousing segments is independent from the wear of the control cams.Moreover, in this structure, the bias forces of the springs counteractthe pressure forces of the compressed gas so that the respectivenecessary force to be applied by the control cams to move the housingsegments is low.

Other actuators than the control cams can be provided for moving thehousing segments, for example fluid-operated adjustment cylinders thatmay also be double action cylinders, whereby the provision of biassprings is obsolete.

Further advantageous embodiments and developments of the inventionresult from the dependent claims, as well as from the description takenin conjunction with the drawings. The following is a description of theinvention with reference to two preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Figures:

FIG. 1 is a sectional view of a housing of a first embodiment of thescrew-type compressor, the section being defined by the rotor axes of aprimary and a secondary rotor,

FIG. 2 illustrates a section through the housing in FIG. 1 taken alongline II--II in FIG. 1,

FIG. 3 illustrates a section through the housing in FIG. 1 taken alongthe line III--III in FIG. 1,

FIG. 4 illustrates a section through the housing in FIG. 1 taken alongthe line IV--IV in FIG. 1,

FIG. 5 illustrates a section through the housing in FIG. 1 taken alongthe line V--V in FIG. 1,

FIG. 6 illustrates a section through a housing according to a secondembodiment of a screw-type compressor corresponding to the sectionalview of FIG. 2, and

FIG. 7 illustrates the housing of FIG. 6 with a housing segmentdisplaced with respect to the state shown in FIG. 6 by rotation of a camshaft.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a housing shell 10 of a housing 12 of a first embodiment ofthe present screw-type compressor 14. The housing shell 10 has asubstantially cylindrical circumferential wall 16 enclosing, in crosssection, two parallel overlapping circular cylinders, the wall beingprovided at the intake end of the screw-type compressor 14 with ahousing cover 18 integrally molded with the circumferential wall 16. Thehousing shell 10 has a inner housing wall 20, the shape of which followsthe envelope of two rotors to be installed in the housing 12. Therotors, not illustrated in the FIGS. 1-7, are each supported in thehousing at the intake side by means of a respective plain bearinglocated in bearing recesses 22, 24 of the housing cover 18. Whenmounted, the rotors take a fitting position represented in the drawingsby the position of the longitudinal axes 26, 28 of the primary andsecondary rotors.

At the pressure side end averted from the housing cover 18, the housingshell has a flange 30 provided with threaded bores 32 that allow thefixing of the housing shell 10 at a pressure side connector block (notillustrated). The connector block may also be designed as the connectorblock of a twin screw compressor having a total of four rotors.

The housing shell 10 that is pot-shaped to receive the primary andsecondary rotors, has its bottom provided with a base 34 extending inparallel to the longitudinal axis 26 of the primary rotor and thelongitudinal axis 28 of the secondary rotor.

To generate compressed gas in the screw-type compressor 14, the toothflanks of the non-illustrated rotors together with the inner housingwall 20 form compression chambers, respectively. Since the inner housingwall 20 follows the course of the envelope along the section line of twocylinders of the rotors, two sealing edges 36 are formed between therotors, extending in the longitudinal direction thereof (FIG. 2).

At the bottom of the housing shell 10 facing the base 34, housingsegments 38, 40, 42, 44 are arranged in a well 46 in the vicinity of thebase 34. The rotor-side top faces 48, 50, 52, 54 of the housing segments38, 40, 42, 44, shown in top plan view in FIG. 1, have a shape thatimitates the course of the envelope of the rotors (FIG. 2) so that theinner housing wall 20, together with the top faces 48, 50, 52, 54 of thehousing segments 38, 40, 42, 44, in a sealing manner circumferentiallyencloses two meshing rotors outside their contact area. Only at theintake end of the housing shell 10, an annular groove 56 extendingthroughout the interior of the housing is provided (FIG. 4), the innercross section of which is wider than the diameter of the envelope of therotors. The annular groove 56 extends only over about a quarter of thelength of the rotors and has a control edge 58 beneath the rotors thatdelimits the region of the inner wall enclosing the rotors in a sealingmanner from the annular groove 56. When the tooth head of a rotor toothhas passed the control edge 58, there is a sealing between the innerhousing wall 20 and the respective tooth flanks so that a return flow ofpressure gas is prevented.

In the screw-type compressor 14 illustrated, the housing segments 38,40, 42, 44 may be displaced within the well 46 in the direction of thedouble arrow B (FIG. 2). When all housing segments 38, 40, 42, 44 are intheir upper end position (FIG. 5), a feed channel with an inlet socket60 and the annular groove 56 from an inlet channel 62 (FIG. 2) thatprovides for air supply to the rotors. When all housing segments 38, 40,42, 44 are in their upper end positions, the compression starts as soonas a tooth of a rotor passes the control edge 58. If, however, onehousing segment, e,g, the housing segment 38, is lowered (FIG. 2),compression is started only when a tooth has passed the edge 64 of thehousing segment 38, since in the lowered state, the inlet channel 62 isformed, downstream of the annular groove 56, also with a return flowchannel 66 formed between the rotors and the top face 48 of the housingsegment 38.

When all housing segments 38, 40, 42, 44 are in their upper endpositions, the screw-type compressor operates at its structurallydefined maximum internal compression ratio (internal pressure ratio).If, however, the first housing segment 38 is lowered completely, thisreduces the internal compression ratio by 15%. If the housing segments40, 42 and 44 are lowered successively, thereby extending the inletchannel 62, the internal compression ratio and, thus, the displacementare reduced correspondingly.

The housing segments 38, 40, 42, 44 are each plate-like bodies withguiding blocks 68, 70 at their small faces, which each enclose a guidingcylinder 72, 74 fixedly arranged at the housing, the blocks beingslidably supported at the cylinder. The guiding blocks 68, 70 each havea top stop surface 76, 78 with which the housing segment abuts againstabutment surfaces 80, 82 of the housing 12 when in the upper endposition. The abutment surfaces 80, 82 each extend in the direction ofthe rotor axes over the width of the four housing segments 38, 40, 42,44. Each housing segment 38, 40, 42, 44 of the first embodiment ispushed to the respective top end position by a first and second spiralspring 84, 86, each arranged enclosing the respective guiding cylinder72, 74.

To move the housing segments 38, 40, 42, 44, a cam shaft 96 with controlcams 88, 90, 92, 94 is provided that extends in parallel to thelongitudinal axis of the primary and secondary rotors 26, 28 and may berotated by means of a stepper motor to displace the housing segments 38,40, 42, 44 into their open position, as indicated by the arrow D in FIG.5. To make it possible for the control cams 88, 90, 92, 94 to applyforce at the housing segments 38, 40, 42, 44, the housing segments 38,40, 42, 44 each have a recess 98 into which a control pin 100 contactingthe respective control cam 88, 90, 92, 94 projects. The control pin 100is arranged on the side of the recess 98 of each housing segment 38, 40,42, 44 that is remote from the rotors so that a rotation of the camshaft 96, while a control pin contacts a control cam 88, 90, 92, 94,causes a displacement of the respective housing segment 38, 40, 42, 44against the bias of the respective first and second spiral spring 72,74.

The embodiment shown in FIGS. 6 and 7 differs from the embodimentillustrated in the FIGS. 1 to 5 only by the structure of the actuatingmeans for the housing segments. For reasons of simplicity, thedescription of FIGS. 6 and 7 uses reference numerals incremented by 100with respect to the reference numerals used for the first embodiment. Toavoid repetitions, reference is made to the description of thecorresponding parts in the first embodiment.

Whereas in the first embodiment, the first and second spiral springs 84,86 push the housing segments into their closing position, correspondingfirst and second spiral springs 184', 186' of the second embodiment pushthe respective housing segment 138' into its opening position.Accordingly, the control cams 188 of the second embodiment are closingcams in contrast to the first embodiment, the cams releasing a controlpin provided in a recess 198' of a respective housing segment 138' uponrotation of the cam shaft 196 such that the corresponding housingsegment is displaced into its opening position illustrated in FIG. 6.

Although a preferred embodiment of the invention has been specificallyillustrated and described herein, it is to be understood that minorvariations may be made in the apparatus without departing from thespirit and scope of the invention, as defined the appended claims.

I claim:
 1. A variable displacement screw-type compressor comprising atleast one primary rotor and at least one secondary rotor arranged in acommon housing (12), said primary and secondary rotors being in meshwith each other for conveying a medium under compression from an inletchannel (62) to an outlet channel, the inlet channel (62) beingdelimited by at least one housing segment (38, 40, 42, 44) slidable inthe housing (12), a sealing side of the segment facing the rotors andfollowing a flow envelope defined by the rotors, said at least onehousing segment (38, 40, 42, 44) being guided and slidable in a well(46) disposed transverse to axes (26, 28) of said rotors, the housingsegment (38, 40, 42, 44) being biased by a spring (84, 86), and acontrol cam (88, 90, 92, 94) being provided for displacing the housingsegment (38, 40, 42, 44) against the bias of the spring (84, 86).
 2. Thescrew-type compressor as defined in claim 1, wherein each housingsegment has two stop surfaces (76, 76) limiting the movement of eachhousing segment (38, 40, 42, 44) towards the rotors.
 3. The screw-typecompressor as defined in claim 1, wherein the housing is provided withtwo abutment surfaces (80, 82) extending vertically relative to adisplacement direction (B) of the housing segment (38, 40, 42, 44). 4.The screw-type compressor as defined in claim 1, including a pluralityof control cams (88, 90, 92, 94) for a plurality of housing segmentsarranged on a common cam shaft (96).
 5. The screw-type compressor asdefined in claim 1, wherein each control cam (88, 90, 92, 94) presses ahousing segment (38, 40, 42, 44) into an open position against the biasof a spring (84, 86).
 6. The screw-type compressor as defined in claim1, wherein each housing segment (38, 40, 42, 44) has a length in theaxial direction of the rotors that corresponds to the distance betweentwo contact points of two meshing pairs of teeth of said rotors.
 7. Thescrew-type compressor as defined in claim 6, wherein at least twohousing segments (38, 40, 42, 44) are arranged successively andadjoining in the axial direction of the rotors.
 8. The screw-typecompressor as defined in claim 7, wherein the housing is provided withtwo abutment surfaces (80, 82) extending vertically relative to adisplacement direction (B) of the housing segment (38, 40, 42, 44). 9.The screw-type compressor as defined in claim 6, wherein each housingsegment has two stop surfaces (76, 76) limiting the movement of eachhousing segment (38, 40, 42, 44) towards the rotors.
 10. The screw-typecompressor as defined in claim 6 wherein the housing is provided withtwo abutment surfaces (80, 82) extending vertically relative to adisplacement direction (B) of the housing segment (38, 40, 42, 44). 11.The screw-type compressor as defined in claim 1, wherein at least twohousing segments (38, 40, 42, 44) are arranged successively andadjoining in the axial direction of the rotors.
 12. The screw-typecompressor as defined in claim 11 wherein each housing segment has twostop surfaces (76, 76) limiting the movement of the housing segment (38,40, 42, 44) towards the rotors.
 13. The screw-type compressor as definedin claim 11, wherein the housing is provided with two abutment surfaces(80, 82) extending vertically relative to a displacement direction (B)of the housing segment (38, 40, 42, 44).